Cosmic Magnetism

Magnetism has been fundamental for travelling and exploring our planet, with the Earth’s magnetic field guiding birds, bees and compass needles.

Furthermore, the effect of the Earth’s magnetic field on charged particles from the Sun has both shielded us from their harmful effects and entranced us with the beautiful aurorae lighting up the northern and southern polar skies.

Through decades of astrophysical research, we have established that magnetism is ubiquitous in our Universe, with interstellar gas, planets, stars and galaxies all showing the presence of magnetic fields. Generating magnetic fields on such large physical scales cannot be achieved through permanent magnets like those found in school science kits, but instead requires huge densities, volumes or motions of electrically charged material, such as the gas that pervades the Milky Way or the outflows of material from the energetic centres of galaxies.

An image of the Sun’s corona, taken in Nov 1999 by the Transition Region and Coronal Explorer (TRACE) satellite. The giant loops of gas seen arching above the Sun’s surface delineate the patterns made by invisible magnetic fields. TRACE is a mission of the Stanford-Lockheed Institute for Space Research, and is part of the NASA Small Explorer programme.

The challenge and opportunities with magnetism

Cosmic magnetism spans an enormous range in its strength, varying by a factor of a hundred billion billion between the weak magnetic fields in interstellar space and the extreme magnetism found on the surface of collapsed stars. Because these cosmic magnetic fields are all-pervasive, they play a vital role in controlling how celestial sources form, age and evolve.

The challenge in studying cosmic magnetism is that, while stars and galaxies can be seen directly by the light they emit, magnetic fields are invisible even to the largest optical telescopes instead requiring the detection of polarised radiation, radiation which exhibits the effects of magnetic fields.

One such example is synchrotron emission, produced when fast-moving (close to the speed of light) electrons, are trapped in magnetic fields, in a similar manner to the way planets are caught by the Sun’s gravity. If we see a heavenly body emitting synchrotron emission, we know that this object must be magnetic, and we can use its properties to determine how strong its magnetic field is and what direction a compass might point if it were near it.

Not all objects, however, are energetic enough to produce synchrotron emission and in these cases other mechanisms are utilised. The spectrally narrow emission from atoms and molecules can be split by the effects of magnetic fields, into two or more emission ‘lines’. This effect, known as “Zeeman Splitting” after the Dutch Physicist Pieter Zeeman, provides a direct measure of the magnetic environment of the atoms and molecules.

Another remarkable mechanism is when the polarised radiation from a distant object passes though the magnetic field of an intervening object and is altered. This effect, known as “Faraday rotation” after the British Scientist Michael Faraday, provides a measure of the magnetic environment of the intervening object (specifically the plane of polarisation is rotated by an angle proportional to the strength of the magnetic field and the density of the medium).

The difficultly with both these techniques is to truly understand the magnetic fields we are seeing we require many hundreds or even thousands of measurements, in the case of Faraday rotation, many distant galaxies or pulsars, lying directly behind the magnetised gas we want to study – like trying to make an environmental study of a large lake, dipping one’s finger in the water at one location is not enough!

When the polarised radio emission from a background galaxy passes through a foreground cloud of magnetic gas, the emission undergoes Faraday Rotation. This effect can be detected with a radio telescope, and used to measure the strength of cosmic magnetic fields. Reprinted with permission from “Intergalactic Magnetic Fields” by Philipp P. Kronberg, Physics Today, December 2002, p. 40. Copyright 2002, American Institute of Physics.

Since the Square Kilometre Array (SKA) will be so much more sensitive than current telescopes, we can use it to revolutionise the study of magnetic fields in space. If we point the SKA at any part of the sky, we will detect the radio emission from thousands of distant faint galaxies, spread like grains of sand all over the sky. These galaxies will be so closely spaced that we can use the Faraday rotation of their polarised radio emission to make detailed studies of the magnetism of all sorts of foreground objects.

Even if we want to study a relatively small cloud of gas, there will be hundreds of background galaxies whose light shines through it, allowing us to build up a detailed picture of the cloud’s magnetism.

Through the mechanisms of Faraday rotation, Zeeman splitting and measuring the direct affects of magnetic fields on the polarised properties of radiation, we will be able to address many important unanswered questions: What is the shape and strength of the magnetic field in our Milky Way, and how does this compare to the magnetism in other galaxies? Is the Universe itself magnetic, and what role has this had on the formation of individual stars and galaxies? Where and how do the magnetic fields originate?

These are all questions for which the unique and fascinating capabilities of the SKA will provide incredible insight. We know that magnetism surrounds us, but with the SKA, we will transform our understanding of what these magnets look like, where they came from, and what role they have played in the evolving Universe.

The ‘snakes’ are regions of gas where the density and magnetic field are changing rapidly as a result of turbulence. [Technical note: the image shows the gradient of linear polarisation over an 18-square-degree region of the Southern Galactic Plane. – Image credit – B. Gaensler et al. Data: CSIRO/ATCA

Understanding cosmic magnetism

The main platform on which the SKA’s studies of cosmic magnetism will be based will be an All-Sky SKA Rotation Measure Survey, in which a year of observing time will yield Faraday rotation measures (RMs) for compact polarized extragalactic sources, an increase by five orders of magnitude over current data sets, and by three orders of magnitude over what could be accomplished with the Extended Very Large Array (EVLA).

This data-set will provide an all-sky grid of RMs at a spacing of just 20–30 arcsec between sources; many these sources will have redshifts from the Sloan Digital Sky Survey (SDSS) and its successors. This RM grid will be a powerful probe for studying foreground magnetic fields at all redshifts.

Understanding the Universe is impossible without understanding magnetic fields. They fill interstellar space, affect the evolution of galaxies and galaxy clusters, contribute significantly to the total pressure of interstellar gas, are essential for the onset of star formation, and control the density and distribution of cosmic rays in the interstellar medium (ISM).

In spite of their importance, the origin of magnetic fields is still an open problem in fundamental physics and astrophysics. Did significant primordial fields exist before the first stars and galaxies? If not, when and how were magnetic fields subsequently generated? What maintains the present-day magnetic fields of galaxies, stars and planets?

The most powerful probes of astrophysical magnetic fields are radio waves.

Synchrotron emission measures the field strength, while its polarisation yields the field’s orientation in the sky plane and also gives the field’s degree of ordering. Faraday rotation yields a full three-dimensional view by providing information on the field component along the line of sight, while the Zeeman effect provides an independent measure of field strength in cold gas clouds.

However, measuring cosmic magnetic fields is a difficult topic still in its infancy, restricted to nearby or bright objects.

The SKA’s role

Through the unique sensitivity and resolution of the Square Kilometre Array (SKA), the window to The Magnetic Universe can finally be fully opened. Apart from the questions we can pose today, it is important to bear in mind that the SKA will certainly discover new magnetic phenomena beyond what we can currently predict or even imagine.

For the Milky Way and for nearby galaxies and clusters, high-sensitivity mapping with the SKA of polarised synchrotron emission, combined with determinations of rotation measures (RM) for extended emission, for pulsars and for the background RM grid mentioned above will allow us to derive detailed three-dimensional maps of the strength, structure and turbulent properties of the magnetic field in these sources, which can be compared carefully with the predictions of various models for magnetic field generation.

An Aitoff projection of the celestial sphere in Galactic coordinates, showing recently compiled sample of 1203 rotation measures (RMs). Closed circles represent positive RMs, while open circles correspond to negative RMs, in both cases the diameter of a circle proportional to the magnitude of its RM. The 887 blue sources represent RMs toward extragalactic sources, while the 316 red sources indicate RMs of radio pulsars. The SKA will be able to measure in excess of ten million RMs, spaced at less than an arcminute between sources. Figure courtesy of Jo-Anne Brown.

At intermediate redshifts, polarised emission from galaxies will often be too faint to detect directly, but the magnetic fields of these sources can be traced by the RMs they produce in the polarised background grid. This will allow detailed studies of the magnetic field configuration of individual objects at earlier epochs; comparison with studies of local galaxies will allow us to understand how magnetised structures evolve and amplify as galaxies mature.

The Faraday rotation in the Andromeda galaxy (M31) has a negative sign on the northeastern side (on the left in the image) but is positive on the opposite side. This proves that the magnetic field in M31 is highly ordered and forms a ring, pointing away from us in the northeast and towards us on the southwest side. This demonstrates the capacity of Faraday rotation to detect fields and determine their strength and direction. The SKA will be able to apply this technique out to high redshifts, encompassing millions of galaxies and even the intergalactic medium.

Furthermore, from a statistical standpoint, the large number of RMs obtained from intervening galaxies and Ly-alpha absorbers will allow us to distinguish between competing models for galaxy and magnetic field evolution as a function of redshift.

At yet higher redshifts, we will take advantage of the sensitivity of the deepest SKA fields, in which we expect to detect the synchrotron emission from the youngest galaxies and proto-galaxies. RMs of the most distant polarised objects (e.g., gamma-ray bursts and quasars beyond the epoch of re-ionisation) can constrain magnetic field strengths at the earliest epoch of galaxy formation, and help distinguish between primordial and seed origins for present-day magnetic fields.

Using the unique sensitivity of the SKA, it may even be feasible to measure Faraday rotation against the Cosmic Microwave Background produced by primordial magnetic fields.

Fundamental to all these issues is the search for magnetic fields in the intergalactic medium (IGM). All empty space may be magnetised, either by outflows from galaxies, by relic lobes of radio galaxies,
or as part of the cosmic web structure. Such a field has not yet been detected, but its role as the likely seed field for galaxies and clusters, plus the prospect that the IGM field might trace and regulate structure formation in the early Universe, places considerable importance on its discovery.

This all-pervading cosmic magnetic field can finally be identified through the all-sky RM survey proposed above. Just as the correlation function of galaxies yields the power spectrum of matter, the analogous correlation function of this RM distribution can then provide the magnetic power spectrum of the intergalactic medium (IGM) as a function of cosmic epoch and over a wide range of spatial scales. Such measurements will allow us to develop a detailed model of the magnetic field geometry of the IGM and of the overall Universe.

In summary, the sheer weight of RM statistics from the SKA, combined with deep polarimetric observations of individual sources, will allow us to characterize the geometry and evolution of magnetic fields in galaxies, clusters and the IGM from high redshifts through to the present, to determine whether there is a connection between the formation of magnetic fields and the formation of structure in the early Universe, and to provide solid constraints on when and how the first magnetic fields in the Universe were generated.

Interesting fact

On Earth our magnetic field helps to channel the stream of particles from the Sun, known as the solar wind to the poles, forming the aurora seen in high northern and southern latitudes. These aurorae have also been observed on the outer gas giant planets as far out as Neptune, showing just how much of an influence the Solar wind has.